Nature Physics doi:10.1038/nphys1457; published online: 22 November 2009; corrected online: 2 December 2009.

In the original article published online, all references to '4 μm downstream' should have been '12 μm downstream'; and in Fig. 3e,f the horizontal scale bar should have been '4 μm'. These changes have been made in all versions of the Letter.